Reaction injection molding (RIM) is a low pressure one-step or one-shot injection of liquid components into a closed mold where rapid polymerization occurs resulting in a molded plastic product. RIM differs from injection molding in a number of important respects. Injection molding is conducted at pressures of about 10,000 to 20,000 psi in the mold cavity by melting a solid resin and conveying it into a mold maintained at room temperature and the molten resin at about 150.degree. to 350.degree. C. At injection temperature of about 150.degree. to 350.degree. C., viscosity of the molten resin in an injection molding process is generally in the range of 50,000 to 1,000,000 and typically about 200,000 cps. In injection molding process, solidification of the resin occurs in about 10 to 90 seconds, depending on the size of the molded product, following which, the molded product is removed from the mold. There is no chemical reaction taking place in an injection molding process when the resin is introduced into a mold.
In a RIM process, viscosity of the materials fed to a mold is about 50 to 10,000 cps, preferably about 1500 cps, at injection temperatures varying from room temperature for urethanes to about 150.degree. C. for lactams. Mold temperatures in a RIM process are in the range of about 100.degree. to 200.degree. C. and pressures in the mold are generally in the range of about 50 to 150 psi. At least one component in the RIM formulation is a monomer that is polymerized to a polymer in the mold. The main distinction between injection molding and RIM resides in the fact that in RIM, a chemical reaction takes place in the mold to transform a monomer to a polymeric state. For practical purposes, the chemical reaction must take place rapidly in less than about 2 minutes for smaller items.
Although urethanes are the only commercial materials currently available for RIM processing, systems based on the use of nylons are being developed due to serious disadvantages of the urethane systems. Among the significant advantages of the nylon systems over the urethanes include the fact that nylons do not require mold release nor off-line painting.
Polymerization of a lactam to give a nylon, i.e., a polyamide, has been known for many years. The earliest processes for this polymerization were slow, requiring several hours, and involved the use of water or acidic reagents as polymerization catalysts. Subsequent work showed that anhydrous lactam could be polymerized above 200.degree. C. in the presence of strongly basic materials, particularly the alkali and alkaline earth metals, their hydrides, hydroxides, alkoxides, oxides, alkyls or amides. More recently, it has been disclosed that the base-catalyzed polymerization of a lactam can be accellerated by the addition of certain compounds that function as promoters. Particularly effective promoters which have been disclosed include acylating agents such as acyl halides, anhydrides and the like; isocyanates and compounds containing tertiary nitrogen having at least two of the three substituents on the nitrogen atom consisting of carbonyl, thiocarbonyl, sulfonyl, phosphenyl, thiophosphenyl and nitroso radicals.
There are a number of pertinent prior art references that relate to the subject matter disclosed herein. U.S. Pat. No. 3,396,145 to Gruenwald discloses epoxy resins cured with 5 to 10% of a lactam and 15 to 65% of a polyfunctional aliphatic acid or anhydride that have good low and high temperature flexibility. U.S. Pat. No. 3,880,948 to Chompff discloses high impact nylon compositions that are prepared by blending nylon with a reaction product of a carboxyl terminated reactive liquid polymer and an epoxy resin. U.S. Pat. No. 3,763,077 to Troy et al discloses anionic polymerization of caprolactam in presence of a polymethylene polyphenyl isocyanate promoter and a polyoxyalkylene polyol plasticizer to form a high impact nylon. U.S. Pat. No. 3,366,608 to Lincoln et al describes polymerization of caprolactam in the presence of an alkaline catalyst, a diacyl biscaprolactam and triethylene tetramine to produce nylon of improved impact strength. The Hedrick et al. U.S. Pat. Nos. 3,944,629, 4,031,163 and 4,034,015 relate to preparation of terpolymers of a lactam, a polyol and an acyl or polyacyl polylactam by anionic polymerization to form nylon of improved impact strength. General Electric has at least one patent on an all-epoxy RIM system wherein reaction proceeds by cationic polymerization.